Maybe the selfharm is your way to deal with your anger. Psychologists say, that for men it is typical to deal with their anger by getting aggressiv and acting out against others. For women it is more typical to take their anger and hold it against themselfs. Like when someone inflicted emotional pain on a woman, she will probably find a way to make it her own fault ( which doesn't meen that women can't be at fault but still ...).
This kind of behaviour does not just happen to women, there are also men who react that way, it is just a bit more typical for women.So maybe you take your anger and make it to self-hatred.So if you feel like blowing up anytime soon, you should probably start to let off steam, not by hurting yourself but by realizing who is making you angry and how could you make them stop. The best way to do that would be to talk to those people or avoid them and maybe the situations which make you angry.And just because you can't really show your emotions that well, dosn't meen you are emotionless, so don't let those words hurt you.
I hope it help a little. Best wishes M